Timbre is generally defined as the tonal identity of a sound. It is the attribute that distinguishes a sound from other sounds of the same pitch and intensity. While the term is most commonly used in a musical connotation, timbre is important in other ways because it is a fundamental aspect of the importance of a sound in the hierarchy of threat or alarm.
In the presentation of music, it can be far more important to quickly identify what the sound is than where it is. This distinction is both intellectual and intuitive; intellectually, timbre is critical to being able to unravel the musical texture in order to understand it. Intuitively, timbre is a fundamental input to the limbic nervous system which is the seat of emotional response. If timbre cannot be quickly perceived, then the musical texture cannot be decoded, nor can an emotional response be elicited. Conscious effort to “understand” the sound impedes the possibility of viscerally reacting to it. The ability to viscerally react to music is an important element of therapeutic effectiveness in music therapy. Basically, improvement in timbre perception allows the conscious thought process to be bypassed.
When a recording is made with the microphones or the performers (or both) in motion, upon playback musical timbre can be more quickly identified. It is hypothesized that this is due to an interaction with human hearing which allows a spatial average energy spectrum to be developed by a process which is in lieu of, or possibly in addition to, the usual averaging of reflections by the human neurophysiological system.
This effect is particularly apparent in headphone (binaural) reproduction. Presumably this is because in normal (non-headphone) listening to either live or reproduced sound, there are small head motions of the listener constantly occurring. And with loudspeakers, even though listener's head may be able to make small movements, the source of the sound is fixed. This may enable the listener to develop the aforementioned spatial average estimate of the energy spectrum. In headphone listening, however, this mechanism is not available because there is no relative motion possible between the listener's ears and the sound source. There also are several other problems associated with binaural presentation, chief among which is the sensation that the sound image is in the middle of one's head. Also there are questions concerning the basic frequency response as it relates to diffuse-field versus direct field equalization.
Accordingly, what is needed is a method to process audio signals to restore or simulate this perceptual mechanism with the use of headphones or loudspeakers.